The present invention relates to a method of making electrodes for use in secondary electrochemical cells. More particularly, it concerns a method of making a negative electrode including a lithium-aluminum alloy and an additive which exhibits an increased loading density and is independent of the ductility or brittleness of the electrode-active material.
A substantial amount of work has been done in the development of high-temperature, secondary electrochemical cells. Positive electrodes for these cells have included chalcogens such as sulfur, oxygen, selenium or tellurium as well as their transition metal chalcogenides. Positive electrode materials such as the sulfides of iron, cobalt, nickel and copper are of current interest.
In high temperature cells, current flow between electrodes often is transmitted by molten electrolytic salt. Particularly useful salts include compositions of the alkali metal halides and/or the alkaline earth metal halides ordinarily incorporating a salt of the negative electrode reactant metal, e.g. lithium. In cells operating at moderate temperatures, aqueous and organic base electrolytes are permissible and these also can include cations of the negative electrode metal.
Alkali metals such as lithium, sodium, potassium or alkaline earth metals including calcium, magnesium, etc. and alloys of these materials are contemplated as negative electrode reactants. Alloys of these materials such as lithium-aluminum, lithium-silicon, lithium-magnesium, calcium-magnesium, calcium-aluminum, calcium-silicon and magnesium-aluminium have been investigated to maintain the negative electrode in solid form and thereby improve retention of the active material at high cell operating temperatures.
One manner of preparing the electrode material is to first form a molten alloy of iron and aluminum. The melt is formed at a temperature above the melting point of aluminum but below the melting point of iron, and temperatures of about 1200.degree. C. or above are satisfactory. The melt is solidified and comminuted to particles of aluminum-iron alloy in the specified proportions. The particles of alloy are integrated into a porous electrically conductive structure. This can be achieved by compacting the particles together with electrolyte, by vibrating or otherwise distributing the particles into a porous electrically conductive substrate or by bonding the particles with a carbonaceous, thermosetting material to form a porous electrically conductive substrate.
Other metallurgical techniques can also be employed to provide alloy compositions. The materials may be melted together and cast or extruded into wire form. Extruded wires or elongated particles of the iron-aluminum alloy can be entangled into a porous mass and compacted. Also, a mass of wires or particles can be sintered to integrate them into a porous substrate in the desired constituent proportions. In one other method, foam metals of the preferred composition can be provided using conventional techniques such as by agitating a molten alloy into a foam by quick solidification.
Cold and hot pressing are also used to fabricate electrodes. Cold pressing depends upon material ductility to obtain desired electrode loading-density. Electrodes having greater than about 48 atom percent lithium-aluminum alloy cannot be made by cold pressing because the lithium alloy particles are too brittle. Cold pressing is generally limited to flat, rectangularly shaped electrodes, whereas advanced electrode design may require annular or other shapes.
The present inventors have found that forming a slurry of a carrier which is chemically inert with respect to the electrode-active material and particulate electrode-active material to a consistency or viscosity such that the slurry is extrudable, that is somewhat like wet sand, results in unexpected high loading densities and the ability to formulate electrodes of complicated shapes.